I have a collection called 'Profiles', and it has a structure like this:
{
_id : XXXXX,
classrooms:{
owner:[],
students: [XYSk0,CCD7U],
},
...
}
I want to get profiles that has classrooms.students equals to CCD7U.
I have tried using $elemMatch on the collection publish function, but unsuccessful:
Meteor.publish('StudentsInClassrooms', function(CCD7U){
return Profiles.find({ classrooms : { $elemMatch :{ students : CCD7U }} });
});
I even tried dot notation, but still unsuccessful:
Meteor.publish('StudentsInClassrooms', function(CCD7U){
return Profiles.find({ 'classrooms.students' : CCD7U });
});
How to properly query that kind of nested collection?
You should use the "$in" operator for such query.
Profiles.find({ 'classrooms.students' : { $in :['CCD7U'] } } )
Ref: MongoDB documentation; $in
When you normally query something you would do it
Profiles.find( { 'classrooms': 'classroomName' } );
But because your are trying to find something from nested you need to do it like this
Profiles.find( { 'classrooms': { 'students' : CCD7U } } );
Related
I'm new in MongoDB.I have read almost all relative posts but cannot find a proper answer for my case. Suppose you have the document:
{ "_id" : "5ad1..",
"linkedin" : { "linkedinId" : "NB..J", "username" : "dolguldur",
"followers":[{name:"John",email:"john#example.com},{name:"Elsa",email:"elsa#example.com}]
}
}
If a follower changes his email, how can i update his record with the new one;
Use the $elemMatch operator to select your desired nested object :
db.collection.update({
followers: {
$elemMatch: {
name: _follower_name,
email: _follower_email
}
}
}, {
"followers.$.email": new_mail
});
The $ positional operator will select the first nested object that match the filter in $elemMatch and update it.
I am using aggregation with mongoDB now i am facing a problem here, i am trying to match my documents which are present in my input array by using $in operator. Now i want to know the index of the lement from the input array now can anyone please tell me how can i do that.
My code
var coupon_ids = ["58455a5c1f65d363bd5d2600", "58455a5c1f65d363bd5d2601","58455a5c1f65d363bd5d2602"]
couponmodel.aggregate(
{ $match : { '_id': { $in : coupons_ids }} },
/* Here i want to know index of coupon_ids element that is matched because i want to perform some operation in below code */
function(err, docs) {
if (err) {
} else {
}
});
Couponmodel Schema
var CouponSchema = new Schema({
category: {type: String},
coupon_name: {type: String}, // this is a string
});
UPDATE-
As suggested by user3124885 that aggregation is not better in performance, can anyone please tell me the performance difference between aggregation and normal query in mongodb. And which one is better ??
Update-
I read this question on SO mongodb-aggregation-match-vs-find-speed. Here the user himself commented that both take same time, also by seeing vlad-z answer i think aggregation is better. Please if anyone of you have worked on mongodb Then please tell me what are your opinion about this.
UPDATE-
I used sample json data containing 30,000 rows and tried match with aggregation v/s find query aggregation got executed in 180 ms where find query took 220ms. ALso i ran $lookup it is also taking not much than 500ms so think aggregation is bit faster than normal query. Please correct me guys if any one of you have tried using aggregation and if not then why ??
UPDATE-
I read this post where user uses below code as a replacement of $zip SERVER-20163 but i am not getting how can i solve my problem using below code. So can anybody please tell me how can i use below code to solve my issue.
{$map: {
input: {
elt1: "$array1",
elt2: "$array2"
},
in: ["$elt1", "$elt2"]
}
Now can anyone please help me, it would be really be a great favor for me.
So say we have the following in the database collection:
> db.couponmodel.find()
{ "_id" : "a" }
{ "_id" : "b" }
{ "_id" : "c" }
{ "_id" : "d" }
and we wish to search for the following ids in the collections
var coupons_ids = ["c", "a" ,"z"];
We'll then have to build up a dynamic projection state so that we can project the correct indexes, so we'll have to map each id to its corresponding index
var conditions = coupons_ids.map(function(value, index){
return { $cond: { if: { $eq: ['$_id', value] }, then: index, else: -1 } };
});
Then we can then inject this in to our aggregation pipeline
db.couponmodel.aggregate([
{ $match : { '_id' : { $in : coupons_ids } } },
{ $project: { indexes : conditions } },
{ $project: {
index : {
$filter: {
input: "$indexes", as: "indexes", cond: { $ne: [ "$$indexes", -1 ] }
}
}
}
},
{ $unwind: '$index' }
]);
Running the above will now output each _id and it's corresponding index within the coupons_ids array
{ "_id" : "a", "index" : 1 }
{ "_id" : "c", "index" : 0 }
However we can also add more items in to the pipeline at the end and reference $index to get the current matched index.
I think you could do it in a faster way simply retrieving the array and search manually. Remember that aggregation don't give you performance.
//$match,$in,$and
$match:{
$and:[
{"uniqueID":{$in:["CONV0001"]}},
{"parentID":{$in:["null"]}},
]
}
}])
In my mongodb (using Mongoose), I have story collection which has comments sub collection and I want to query the subdocument by client id, as
Story.find({ 'comments.client': id }, { title: 1, 'comments.$': 1 }, function (err, stories) {
...
})
})
The query works except that it only returns the first matched subdocument, but I want it to return all matching subdocuments. Did I miss an option?
EDIT:
On Blakes Seven's tip, I tried the answers from Retrieve only the queried element in an object array in MongoDB collection, but I couldn't make it work.
First tried this:
Story.find({'comments.client': id}, { title: 1, comments: {$elemMatch: { client: id } } }, function (err, stories) {
})
It also returns the first match only.
Then, I tried the accepted answer there:
Story.aggregate({$match: {'comments.client': id} }, {$unwind: '$comments'}, {$match : {'comments.client': id} }, function (err, stories) {
})
but this returns nothing. What is wrong here?
UPDATE:
My data structure looks like this:
{
"_id" : ObjectId("55e2185288fee5a433ceabf5"),
"title" : "test",
"comments" : [
{
"_id" : ObjectId("55e2184e88fee5a433ceaaf5"),
"client" : ObjectId("55e218446033de4e7db3f2a4"),
"time" : ISODate("2015-08-29T20:16:00.000Z")
}
]
}
I want to execute map-reduce operation on specific documents from MongoDB.
Let's say, we have this "document":
{
a: {
b: {
c:{}
}
}
}
I want to select only those documents, that have an a.b.c field. From this documentation page I know, that $exists operator can help.
To check, that a exists I should use:
query: {
a: { $exists : true }
}
But what query should I use for nested field, described before?
query: {
//?
}
This isn't related to the fact of using map-reduce.
query: {
'a.b.c' : { $exists : true }
}
So is a standard query on an embedded document:
http://docs.mongodb.org/manual/tutorial/query-documents/#embedded-documents
What I'm trying to do is pretty straightforward, but I can't find out how to give one field the value of another.
I simply want to update one field with the character count of another.
db.collection.update({$exists:true},{$set : {field1 : field2.length}})
I've tried giving it dot notation
db.collection.update({$exits:true},{$set : {field1: "this.field2.length"}})
As well as using javascript syntax
db.collection.update({$exits:true},
{$set : {field1: {$where : "this.field2.length"}})
But just copied the string and got a "notOkforstorage" respectively. Any help?
Update:
I only get the "notOkforStorage" when I query by ID:
db.collection.update({_id:ObjectID("38289842bbb")},
{$set : {field1: {$where :"this.field2.length"}}})
Try the following code:
db.collection.find(your_querry).forEach(function(doc) {
doc.field1 = doc.field2.length;
db.collection.save(doc);
});
You can use your_querry to select only part of the original collection do perform an update. If you want to process an entire collection, use your_querry = {}.
If you want all operations to be atomic, use update instead of save:
db.collection.find( your_querry, { field2: 1 } ).forEach(function(doc) {
db.collection.update({ _id: doc._id },{ $set: { field1: doc.field2.length } } );
});
Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update/creation of a field based on another field:
// { "_id" : ObjectId("5e84c..."), "field1" : 12, "field2" : "world" }
db.collection.update(
{ "_id" : ObjectId("5e84c...") },
[{ $set: { field1: { $strLenCP: "$field2" } } }]
)
// { "_id" : ObjectId("5e84c..."), "field1" : 5, "field2" : "world" }
The first part {} is the match query, filtering which documents to update.
The second part [{ $set: { field1: { $strLenCP: "$field2" } } }] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline). $set is a new aggregation operator and an alias for $addFields. Any aggregation operator can be used within the $set stage; in our case $strLenCP which provides the length of field2.
As far I know the easiest way is the read and write aproach:
//At first, get/prepare your new value:
var d= db.yourColl.fetchOne({....});
d.field1== d.field2.length;
// then update with your new value
db.yourColl.save(d);
Your are using exists in the wrong way.
Syntax: { field: { $exists: <boolean> } }
You use of $where is also incorrect
Use the $where operator to pass either a string containing a JavaScript expression or a full JavaScript function to the query system
db.myCollection.find( { $where: "this.credits == this.debits" } );
db.myCollection.find( { $where: "obj.credits == obj.debits" } );
db.myCollection.find( { $where: function() { return (this.credits == this.debits) } } );
db.myCollection.find( { $where: function() { return obj.credits == obj.debits; } } );
I think you should use Map-Reduce for what you are trying to do.